1. Field of the Invention
The invention relates to a display device comprising a cathode ray tube comprising an in-line electron gun for generating at least one electron beam, a phosphor screen on an inner surface of a display window, a deflection means for deflecting the electron beams and an electromagnetic means for generating a magnetic field to dynamically influence the path or shape of the electron beams as a finction of a position where the electron beam lits the phosphor screen, and means for supplying a current to the electromagnetic means.
2. Description of the Relate Art
Such a device is known from PCT-application no. WO99/34392-A1. The known device comprises a color display device comprising a deflection unit for deflecting the electron beams across a color selection electrode. The known device also comprises a pair of means, which are arranged at some distance from each other so as to dynamically influence the trajectories (paths) of the electron beams and to decrease the distance between the electron beams at the location of the deflection plane as a function of the deflection. By doing so, the distance between the electron beams decreases for larger deflection angles. The distance between the color selection electrode and the phosphor screen is inversely dependent on the distance between the electron beams. Thus, a decrease of the distance between the electron beams enables the distance between the color selection electrode and the phosphor screen to be increased.
WO99/34392-A1 discloses a design in which the electromagnetic means are arranged in series with the deflection coils. Such direct driving has the disadvantage that the co-operating means draw their power directly from the deflection circuit and are coupled directly to the deflection circuit. The currents through the co-operating means will be temperature-dependent, i.e. as the temperature of the co-operating means increases, the current through the co-operating means is influenced, which influences the deflection current. Conversely, a temperature increase of the deflection coils influences the deflection current, thereby influencing the current through the co-operating means. Furthermore, the current through the co-operating means cannot be more than the current through the deflection circuit, putting a limit on the available current through the co-operating means. If the current through the co-operating means is to be smaller than the deflection current, a part of the deflection current will have to bypass the co-operating means, leading to a loss of power.
It is an object of the invention to provide a display device as described in the opening paragraph, having an improved power supply for the electromagnetic means.
To this end, a display device according to the invention is characterized in that the means for supplying a current comprise a transformer arranged in parallel with the line deflection coil, or the means for supplying a current comprise a supply coil wound around the deflection core.
In a device in accordance with the invention, the energy for driving the electromagnetic means is drawn from the deflection circuit. Thus, the supply means can be and preferably is provided on the deflection unit. This removes the need for an external supply means and simplifies the design of the device. In a device in accordance with the invention, the power for driving the electromagnetic means is taken from the deflection circuit without, however, substantially influencing the deflection, as will be explained below.
Preferably, the supply means are arranged in such a way that the power is drawn from the fly-back voltage of the horizontal deflection coil.
In this embodiment, negative effects on the deflection are reduced with respect to embodiments in which the power is drawn from the scan voltage.
These and other objects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.